A great many methods of encoding and decoding an audio signal have been developed up to now. Particularly, in these days, IS13818-7 which is internationally standardized in ISO/IEC is publicly known and highly appreciated as an encoding method for reproduction of high quality sound with high efficiency. This encoding method is called AAC. In recent years, the AAC is adopted to the standard called MPEG4, and a system called MPEG4-AAC that has some extended functions added to the IS13818-7 is developed. An example of the encoding procedure is described in the informative part of the MPEG4-AAC.
Following is an explanation for the audio encoding device using the conventional method referring to FIG. 1. FIG. 1 is a block diagram that shows a structure of the conventional encoding device 100. The encoding device 100 includes a spectrum amplifying unit 101, a spectrum quantizing unit 102, a Huffman coding unit 103 and an encoded data stream transfer unit 104. An audio discrete signal stream in the time domain obtained by sampling an analog audio signal at a fixed frequency is divided into a fixed number of samples at a fixed time interval, transformed into data in the frequency domain via a time-frequency transforming unit not shown here, and then sent to the spectrum amplifying unit 101 as an input signal to the encoding device 100. The spectrum amplifying unit 101 amplifies spectrums included in a predetermined band with one certain gain for each of the predetermined band. The spectrum quantizing unit 102 quantizes the amplified spectrums with a predetermined conversion expression. In the case of AAC method, the quantization is conducted by rounding off frequency spectral data which is expressed with a floating point into an integer value. The Huffman coding unit 103 encodes the quantized spectral data in groups of certain pieces according to the Huffman coding, and encodes the gain in every predetermined band in the spectrum amplifying unit 101 and data that specifies a conversion expression for the quantization according to the Huffman coding, and then sends the codes of them to the encoded data stream transfer unit 104. The encoded data stream that is encoded according to the Huffman coding is transferred from the encoded data stream transfer unit 104 to a decoding device via a transmission channel or a recording medium, and is reconstructed into an audio signal in the time domain by the decoding device. The conventional encoding device operates as described above.
In the conventional encoding device 100, compression capability for data amount is dependent on the performance of the Huffman coding unit 103, so, when the encoding is conducted at a high compression rate, that is, with a small amount of data, it is necessary to reduce the gain sufficiently in the spectrum amplifying unit 101 and encode the quantized spectral stream obtained by the spectrum quantizing unit 102 so that the data becomes a smaller size in the Huffman coding unit 103. However, if the encoding is conducted for reducing the data amount according to this method, the bandwidth for reproduction of sound and music becomes narrow. So it cannot be denied that the sound would be furry when it is heard. As a result, it is impossible to maintain the sound quality. That is a problem.
The object of the present invention is, in the light of the above-mentioned problem, to provide an encoding device that can encode an audio signal with a high compression rate and a decoding device that can decode the encoded audio signal and reproduce wideband frequency spectral data and wideband audio signal.